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Photosynthetic performance and species success

During ecological successions, the differences in physiological performances of co-occurring species contribute to trigger the changes in plant community composition: however, plant physiology and vegetation dynamics are rarely investigated together. A long term rainfall reduction experiment carried out on a Mediterranean shrubland, provided to our research group (Fig. 1) the occasion to analyze the relationships between the photosynthetic performances of three shrub species and the changes in their abundances (Fig.2).

Figure 1 The authors (DL, GdD, GG and PDA) at the experimental site.

Figure 2 Cover trend dynamics of the studied species under natural and manipulated rainfall regime.

The first hypothesis we tested was that the species success depended on the capacity to assimilate more CO2 than the competitors; however, neither leaf-level nor plant-level photosynthesis rates resulted related to the cover degree variation rates of the species (Fig. 3a and Fig. 3b).

Figure 3. Relationship between the different indeces of plant fitness and the species success (measured as cover degree variation rate).

High rates of photosynthesis are in fact important to grow and gain space during the favourable periods, but to persist within the community, plants have also to survive when resources are scarce. During summer, water shortage reduce the CO2 assimilation until determine a negative plant carbon balance (exhaustion phase), and can lead to dieback for carbon starvation. To take into account this process, we quantified the drought impact on plant survival, estimating, for each species and rainfall regime, the length of the exhaustion phase.

All components of plant success were at this point available: the growth capacity during optimal conditions (estimated by the plant-level photosynthesis), and the probability to die during drought stress (estimated by the length of exhaustion phase). Dividing the first one by the second, we finally obtained an index (named PPIred) related to the cover degree variation rates of the species, in both natural and manipulated rainfall regime (Fig. 3c).

In conclusion, these results suggest that this quantitative-mechanistic approach can be used to compare the capacity of co-occurring species to persist within plant communites, also in response to change in environmental conditions (rainfall reduction).

Oikos Journal

Oikos is a journal issued by the Nordic Ecological Society and is one of the leading peer-reviewed journals in ecology. Oikos publishes original and innovative research on all aspects of ecology, defined as organism-environment interactions.